Electroluminescent device



Jan. 15, 1963 l. E. BUCK, JR., ETAL 3,073,982 ELECTROLUMINESCENT DEVICEFiled Dec. 23, 1960 FIG. I.

FIG. 2.

BUFFER-DIELECTRIC LAYER l6 SEMl-CONDUCTOR MATERIAL LAYER l4 IRONSUBSTRATE l2 United States Patent 3,073,982 ELECTROLUMENESCENT DEVICEIvan E. Buck, Jr., East Orange, and Willard M. Pakutka,

Orange, N.J., assignors to Westinghouse Electric Corporation, EastPittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 23, 1960, Ser.No. 78,156 7 Claims. (Cl. 313-408) This invention relates toelectroluminescent devices and, more particularly, to anelectroluminescent device which has improved performancecharacteristics.

The phenomenon of electroluminescence was first disclosed by G.Destriau, one of his earlier publications appearing in London, Edinburghand Dublin Philosophical Magazine, Series 7, volume 38, No. 285, pages700-737 (October 1947). Since this early publication, electroluminescentdevices have been marketed commercially. In one construction forelectroluminescent devices, the phosphor is embedded in a plasticdielectric material. In another construction for such devices, thephosphor is embedded in a glass or ceramic material in the form of alayer and the energizing electric field is applied across this layer toproduce light. Electroluminescent devices which do not utilize plasticmaterial between the spaced electrodes can be categorized asceramic-type electroluminescent devices. The maintenance of light outputfor such ceramic-type devices is normally very good, but the level ofillumination tends to be relatively low.

It is the general object of this invention to provide anelectroluminescent device having improved brightness.

It is another object to provide a ceramic-type electroluminescent devicewhich operates with a very large margin of safety so that a very highvoltage is required to cause an electrical breakdown across the deviceelectrodes.

It is a further object to provide a foundation-electrode and dielectriclayer which can be used as a part of an improved electroluminescentdevice.

The aforesaid objects of the invention, and other objects which willbecome apparent as the description proceeds, are achieved by providingan electroluminescent device comprising a conducting substrate havingcarried thereover a layer of iron-titanate-inclnding, semi-conductormaterial. Coated over the semi-conductor is a buffer-dielectric layerformed of tianate and glass. Over the buffer layer is carried aphosphor-dielectric layer and a light-transmitting electrode is carriedover the phosphor-d electric layer. The ratio of glass to titanate inthe buffer layer is carefully controlled and this layer is applied insuch manner that the layer surface which is adjacent the semi-conductorlayer is rich in glass and the layer surface which is adjacent thephosphor-dielectric layer is substantially free of iron compound.

For a better understanding of the invention, reference should be had tothe accompanying drawing wherein:

FIG. 1 is an elevational view, partly in section, of anelectroluminescent device constructed in accordance with the presentinvention;

FIG. 2 is a fragmentary, sectional-elevational view of thefoundation-electrode and overlaying dielectric layer portion of thedevice as shown in FIG. 1.

With specific reference to the form of the invention shown in thedrawing the electroluminescent device 10 generally comprises aconducting substrate 12 over which is applied a semi-conductor materiallayer 14 which comprises an iron-titanate-containing material. A layer16 comprising a mixture of barium titanate and glass is carried over thelayer 14 and a phosphor-dielectric layer 18 is carried over the bariumtitanate-glass layer 16. A light-transmitting electrode layer 20 iscarried over the phosphor-dielectric layer 18. Preferably a protective,light-transmitting layer 22 which comprises a cover coat is carried overthe electrode layer 20.

As a specific example, the conducting substrate is formed ofconventional enameling iron of 22 gauge thickness. The semi-conductormaterial layer 14 is formed of a titania-precipitating glass having thefollowing percentage by weight composition: Na O, 10.5%; K 0, 3.5%; B 014.0%; SiO 45%; TiO- 20.0%; P 0 2.0%; and F 5.0%. Anytitania-precipitating glass which can be enameled can be substituted forthe fore going specific example and such glasses are very well known.

The buffer-dielectric layer 16 comprises from to by weight of finelydivided barium titanate and from 20% to 5% by Weight of glass which isfused about the barium titanate to form a substantially continuouslayer. The surface of the layer 16 which is adjacent the semi-conductormaterial layer 14 is rich in glass and the surface of the layer 16 whichis proximate the phosphordielectric layer 18 is substantially free ofiron compound. As a specific example, the layer 16 has a thickness ofapproximately 1 mil and comprises 90% by weight of finely divided bariumtitanate and 10% by weight of glass. The percent by weight compositionof the glass which is used in forming the layer 16 is as follows: ZnO,15.4%;

BaO, 31.7%; B 0 19.1%; SiO 4.0%; K 0, 7.7%; Na 0, 3.8%; Li O, 1.3%; A1 03.9%; TiO 11.2%; Sb O 1.9%. Any glass which has a relatively lowsoftening temperature can be substituted for this foregoing example. Theaverage particle diameter of the finely divided barium titanate is abouttwo microns and this is subject to considerable variation.

The phosphor in the layer 18 is preferably embedded in alight-transmitting glass dielectric material. Any suitableelectroluminescent phosphor can be used and as an example, zinc sulfidephosphor which is activated by copper and coactivated by chlorine hasbeen found to be very suitable. The phosphor is finely divided and as anexample, has an average particle diameter of about ten microns. Thisparticle diameter is subject to considerable variation. As a specificexample, the glass in which the phosphor is embedded has the followingformulation, as expressed in percent by weight: SiO 10.1%; B 0 21.5%;TiO 4.9%; ZnO, 29.2%; BaO, 16.3%; CaO, 4.5%; MgO, 1.8%; K 0, 8.3%; Na O,3.4%. Seven parts by weight of glass are used per four parts by weightof phosphor. The weight ratios of phosphor to glass are not critical andthe glass content has been varied from ten parts by weight to 2.5 partsby weight per four parts by weight of phosphor. Even this indicatedrange can be extended.

The light-transmitting, electrically conducting layer 20 is preferablyformed of tin oxide which is deposited in accordance with conventionalpractices. Other known materials such as indium oxide can be substitutedfor the tin oxide electrode. The exterior layer 22, which is carriedover the electrode layer 20, provides electrical insulation andprotection against atmospheric moisture. The layer 22 can be forced ofany suitable enameling 'glass or it can be replaced by a suitablelight-transmitting plastic material, such as epoxy resin. The layer 22can be omitted if desired.

In fabricating the device 10, the enameling iron substrate 12 is placedin a substantially horizontal orientation and a frit of thetitania-precipitating glass is sprayed thereover. The glass frit is veryfinely divided and to facilitate deposition, parts by weight'of thefinely divided glass frit are mixed with 2 parts by weight clay,

. w 3 0.17 part by weight potassium carbonate, and 13.1 parts by weightbarium titanate. These finely divided materials are mixed with 50 partsby weight of water to form a slurry which is sprayed onto the substrate.12. In the preferred construction, the. layer 14 has a thickness ofabout '10 mils and in order to make a layer of this thick-.

mess, it is preferred to form the layer 14 in two steps. In forming thelayer 14, the foregoing coating composition is sprayed onto theenameling iron substrate 12 to a thickness of slightly greater than 4mils. The substrate is then heated to a temperature of 800 C. forapproximately five minutes. The second layer is then sprayed over thefirst layer to a thickness of slightly greater than 6 mils and theforegoing firing repeated. During the indicated firings, which fuse theglass frit into a continuous layer, iron from the substrate 12 reactswith titania which is precipitated from the. glass comprising the layer14 to form an iron-titanate complex. This .iron-titanate complexprovides the 7 layer 14 with the properties of a semi-conductor. Inactual measurements, the electrical resistance, as measured through athreequarter square inch portion of the layer 14, was approximately10,000 ohms. The layer 14 should have a thickness at least equal to thatof the buffer layer 16 and preferably, the layer 1 4 has a thicknessabout ten times that of the buffer layer 16.

In forming the butter layer 16, ninety parts by weight of the finelydivided barium titanate are mixed with ten parts by weight of the finelydividedglass and applied over the layer 14 to a thickness of about onemil by means of a conventional silk screen deposition process. As anexample, a 225 mesh screen is used. The applied glassbarium titanatepowder is then fired at a temperature of 620 C. for a period of fiveminutes. During this firing, the glass apparently settles or isattracted toward the bottom of the layer 16, leaving a small residualportion of the glass, dispersed throughout the remainder of the layer16to fuse the particles of barium titanate to.- gether. If the glass ispresent in amount of more than 20% by weight of the layer 16, thedielectric constant of this layer will be excessively lowered, therebydecreasing the total brightness for the device. If less than by weightof glass is used in forming the layer 16, the finely divided bariumtitanate particles willnot be adequately bonded together. The electricalresistance of. the foregoing bufier or barrier layer 16, as measuredthrough a three-quarter square inch portion, was in the order of onemegohrn. The semi-conductor layer 14 thus has a very low electricalresistance as compared to the butter layer 16.- The foregoingfoundation-electrode and buffer layer is shown in Fig. 2.

In the next manufacturing step, the phosphor-dielectric layer 18 isapplied over the butter. layer 16. This phosphor-dielectric layer ispreferably applied as a powder mixture, using a silk screen with a 163mesh. Alternatively, conventional spraying can be used to deposit thephosphor-dielectric powder. This powder is deposited to a thickness ofapproximately 1 mil. The deposited powdered glass frit and phosphor arethen fired at a temperature of approximately 670 C. for approximatelyfive'minutes. This will form a substantially continuous layer ofphosphor and dielectric. To complete the fabrication of the device, theoverlaying electrode layer 20 is applied in accordance with conventionalpractices, as in the cover coat or protective layer 22.

The foregoing specific device operates with a relatively high brightnesswhen excited with a potential of 120 volts, 60 cycles. If the device hasa total area of approximately 3.5 square inches, the power consumptionis in the order of 0.025 watt, the power factor is about 0.25 and thebrightness is in the order of 1.2 to 1.5 ft. lamberts.

The presence of the bufier layer 16 is extremely important. If thislayer '16 is dispensed with and the phosphor-dielectric layer 18 isplaced directly on the semiconductor material layer 14, the operation ofthe device will be extremely poor. Similar devices so constructed have areasonably good surface brightness, but operate 7 include the bufferlayer, however, displaya total resistance between the electrodes varyingfrom 10 to 10 ohms. The buffer layer 16 thus functions to prevent anymigration of iron, during the firing processes, from the semi-conductormaterial layer 14 to the phosphor-dielectric layer 18. This isattributable to the fact that barium titanate is quite inert withrespect to reacting with iron.

In addition, while the surface of the buffer layer 16 which is adjacentthe semi-conductor material layer 14 is quite rich in glass, the othersurface of the butter layer 16 which is adjacent the phosphor-dielectriclayer contains only a very small amount of glass. Accordingly, the uppersurface of the buffer layer 16 which is adjacent the phosphor-dielectriclayer '18 is substantially free of any iron compound which cancontaminate the phosphordielectric layer. 1

Another requirement for the buffer layer 16 is that it should have anextremely high dielectric constant, in order that the capacitivereactance of this layer is very small. If glass per se is used toreplace the barium titanate in forming the butter layer, the performanceof the device is impaired, since a considerable potential drop willappear across this glass layer due to increased impedance, in additionto the tendency of the usual glass to react with iron.

Eachlayer '14, 16 and 18, of the foregoing device 10 can be expressed asa parallel-connected capacitance and resistance, with eachparallel-connected capacitance and resistance being connected in seriesWith respect to the semi-conductor material layer 14, the capacitivereactance of this layer is quite high as compared to its relatively lowelectrical resistance. The power factor of this semi.- conductor layer14 thus approaches unity and because of the relatively low electricalresistance of the layer 14, there is very little power consumed in thislayer. The electrical characteristics of the layer 14 are such as toplace substantially all applied potential across the layers 16 and 18.The semi-conductor material layer 14 also imparts very high electricalbreakdown characteristics to the device 10, since the layer 14 places inseries with the phosphondielectric layer 18 what amounts to an infinitenumber of resistances. When incipient electrical breakdown starts tooccur across any portion of the layer 18, the additional current whichis drawn immediately quenches such breakdown by virtue of the potentialdrop generated across the semi-conductor layer 14. Thus the electricalbreakdown characteristics of the device 10 are greatly improved by thehigh electrical-breakdown characteristics of the semi-conductor materiallayer 14.

The equivalent circuit for the butter layer 16 can also be expressed asa parallel-connected capacitance and resistance, as indicatedhereinbefore. Because of the extremely high dielectric constant ofbarium titanate, and the predominance of this material, the capacitivereactance across the layer 16 is extremely small and substantially allof the potential which is applied across the layers 16 and 18 appearsacross the phosphor-dielectric layer 18. Since the phosphor-dielectriclayer 18 is extremely thin, a relatively intense electric field isapplied to excite the phosphor.

In the usual plastic-type electroluminescent device intended to operatewith volts excitation, wherein the phosphor is embedded in a plasticmaterial such as polyvinyl chloride, it is desirable to make thephosphor-plastic dielectric layer approximately 2 mils thick, in orderto insure adequate protection against electrical breakdown across thedevice electrodes. Such a device will operate with a surface brightnessof approximately 0.7 to 0.8 ft. lambert and electrical breakdown willnormally be encountered at a potential of approximately 300 volts. Inthe present device, the spacing between the electrodes is approximately12 mils. Because the semi-conductor material layer 14 serves to providebreakdown protection and consumes but very little power due to its lowelectrical resistance, the device operates, when excited with 120 volts60 cycles, with a surface brightness of 1.2 to 1.5 ft. lamberts, asindicated hereinbefore. In addition, the potential required to causeelectrical breakdown between the device electrodes is in the order ofabout 900 volts.

Plastic-type electroluminescent devices have been fabricated with anadditional layer of high-dielectric material in order to provideimproved electrical breakdown characteristics, while maintaining thesurface brightness at a relatively high value. Such plastic-type devicesnormally have relatively poor maintenance of light output, however, andas a general rule, the maintenance of light output of plastic-typeelectroluminescent devices will not be as good as that which is obtainedwith ceramic-type electroluminescent devices. The presentelectroluminescent cell construction can be operated at a high level ofbrightness and with a large margin of safety against electricalbreakdown. In addition, because of the ceramic-type construction, themaintenance of light output for the device is very good. The foregoingspecific example is subject to considerable modification. For example,the electroluminescent phosphor layer can be replaced by a continuousthin film of electroluminescent phosphor which is formed directly onto alight-transmitting electrode. Such a construction is disclosed incopending application S.N. 837,988, filed September 3, 1959 and owned bythe present assignee. As another possible embodiment, the semi-conductormaterial layer 14 can be made much thinner, if desired. This willdecrease somewhat the potential which is required to cause breakdownbetween the electrodes of the device. For some applications, however, alarge degree of breakdown protection is not essential. The additionalbufferdielectric layer 16 can be made thicker if desired, in order toinsure that no possible iron migrates through the layer 16 tocontaminate the phosphor-dielectric layer 18. As a general rule, inorder to provide an adequate barrier against migration of iron to thephosphor layer 18, the thickness of the layer 16 should be at leastabout as great as the thickness of the phosphor layer 18. It should beunderstood that while the foregoing specific device is intended to beoperated with an excitation potential of 120 volts, the device can beoperated with a greatly increased .excitation potential, in order toincrease the brightness.

Finely divided barium titanate is preferred for use in the bufier layer16. Equal amounts of other finely divided materials which have a veryhigh dielectric constant can be substituted therefor. Examples of othersuitable materials are strontium titanate, any solid solution of bariumtitanate and strontium titanate or a physical mixture of barium titanateand strontium titanate in any proportions. Any of these other indicatedmaterials can be substituted for the barium titanate in the foregoingspecific example.

It will be recognized that the objects of the invention have beenachieved by providing an electroluminescent device having improvedbrightness and performance. Such a device can be operated with both highbrightness and a very large margin of safety with respect to prevent ingany tendencies for electrical breakdown across the device electrodes.There has also been provided a foundation-electrode and dielectric layerfor an electroluminescent device.

While a best embodiment has been illustrated and described in detail, itis to be particularly understood that the invention is not limitedthereto or thereby.

We claim: Y

1. A foundation-electrode carrying thereon a dielectric material layerfor use as a component part of an electroluminescent device, saidfoundation-electrode and dielectric material layer comprising: aconducting substrate; a semi-conductor layer comprising aniron-titanate-containing material carried on said conducting substrate;and a mixed glass and titanate layer carried over said semi-conductorlayer; said semi-conductor layer having a very low electrical resistancecompared to the electrical resistance of said glass-titanate layer; saidsemi-conductor layer also having a thickness at least equal to thethickness of said glass-titanate layer; said glass-titanate layercomprising from to 95% by weight of at least one finely divided materialof the group consisting of barium titanate, strontium titanate andbarium-strontium titanate, with from 20% to 5% by weight of glass fusedabout the particles of the material of said group; the surface of saidglasstitanate layer adjacent to said semi-conductor layer being rich inglass and the opposite surface of said glass-titanate layer beingsubstantially free of iron compound.

2. A foundation-electrode carrying thereon a dielectric material layerfor use as a component part of an electroluminescent device, saidfoundation-electrode and dielectric material layer comprising, aconducting substrate, a semi-conductor layer comprising aniron-titanatec0ntaining material carried on said conducting substrate,and a mixed glass and titanate layer carried over said semiconductorlayer, said semi-conductor layer having a very low electrical resistancecompared to the electrical resistance of said glass-titanate layer, saidsemi-conductor layer also having a thickness at least equal to thethickness of said glass-titanate layer, said glass-titanate layercomprising from 80% to 95 by weight of barium titanate particles andfrom 20% to 5% by weight of glass fused about the barium titanateparticles, the surface of said glass-titanate layer adjacent to saidsemi-conductor layer being rich in glass and the opposite surface ofsaid glass-titanate layer being substantially free of iron compound.

3. A foundation-electrode carrying thereon a dielectric material layerfor use as a component part of an electroluminescent device, saidfoundatio-n-electrode and dielectric material layer comprising, aconducting substrate, 'a semi-conductor layer comprising aniron-titanate-containing material carried on said conducting substrate,and a mixed glass and titanate layer carried over said semiconductorlayer, said semi-conductor layer having a very low electrical resistancecompared to the electrical resistance of said glass-titanate layer, saidsemi-conductor layer also having a thickness of about ten times thethickness of said glass-titanate layer, said glass-titanate layercomprising about by weight of barium titanate particles and about 10% byweight of glass fused about the barium titanate particles, the surfaceof said glass-titanate layer adjacent to said semi-conductor layer beingrich in glass and the opposite surface of said glass-titanate layerbeing substantially free of iron compound.

4. An electroluminescent cell comprising: a conducting substrate; asemi-conductor layer comprising an irontit-anate-containing materialcarried on said conducting substrate; a titanate and mixed glass bufferlayer carried on said semi-conductor layer; a layer comprisingelectroluminescent phosphor carried on said buffer layer; and alight-transmitting electrode layer carried on said layer comprisingelectroluminescent phosphor; said semi-conductor layer having a very lowelectrical resistance compared to the electrical resistance of saidbuffer layer; said semi-conductor layer also having a thickness at leastequal to the thickness of said buffer layer; said buffer layercomprising from 80% to by weight of at least one finely divided materialof the group consisting of barium titanate, strontium titanate andbarium-strontium titanate,

7 and with from 20% to by weight of glass fused about the particles ofthe material of said group; the surface of said butler layer adjacent tosaid semi-conductor layer being rich in glass and the surface of saidbuffer layer adjacent to said layer comprising electroluminescentphosphor being substantially free of iron compound.

5. An electroluminescent cell comprising, a conducting substrate, asemi-conductor layer comprising an irontitanate-containing materialcarried on said conducting substrate, a titanate and mixed glass bufferlayer carried on said semi-conductor layer, a layer comprisingelectroluminescent phosphor'carried on said butter layer, and alight-transmitting electrode'layer carried on said layer comprisingelectroluminescent phosphor, said semi-conductor layer having a very lowelectrical resistance compared to the electrical resistance of saidbuffer layer, said semi-conductor layer also having a thickness at leastequal to the thickness of aid bufi'er layer, said butter layercomprising from 80% to 95% by weight of barium titanate particles andwith from 20% to 5% by Weight of glass fused about the barium titanateparticles, the surface of said buffer layer adjacent to saidsemi-conductor layer being rich in glass and the surface of said bufferlayer adjacent to said layer comprising electroluminescent phosphorbeing substantially free or" iron compound.

6. An electroluminescent cell comprising, a conducting substrate, asemi-conductor layer comprising an irontitanate-containing materialcarried on said conducting substrate, a titanate and mixed glass bufferlayer carried on said semi-conductor layer, a layer comprisingelectroluminescent phosphor carried on said butter layer, and alight-transmitting electrode layer carried on said layer comprisingelectroluminescent phosphor, said semi-conductor layer having a very lowelectrical resistance compared to the electrical resistance of saidbutter layer, said semi-conductor layer also having a thickness at leastequal to the thickness of said buffer layer, said buffer layercomprising about by Weight of barium titanate particles and about 10% byweight of glass fused about the barium titanate particles, the surfaceof said butter layer adjacent to said semi-conductor layer being rich inglass and the surface of said buffer layer adjacent to said layercomprising electroluminescent phosphor being substantially free of ironcompound, and said buffer layer having a thickness at least about asgreat as the thickness of said layer comprising phosphor.

7. An electroluminescent cell comprising, a conducting substrate, asemi-conductor layer comprising an irontitanate-containing materialcarried on said conducting substrate, a titanate and mixed glass buflerlayer carried on said semi-conductor layer, a layer comprisingelectroluminescent phosphor carried on said bufier layer, and alighttransmitting electrode layer carried on said layer comprisingelectroluminescent phosphor, said semiconductor layer having a very lowelectrical resistance compared to the electrical resistance of saidbuffer layer, said semi-conductor layer also having a thickness aboutten times the thickness of said buffer layer, said butter layercomprising about 90% by weight of barium titanate particles and about10% by Weight of glass fused about the barium titanate particles, thesurface of said buffer layer adjacent to said semi-conductor layer beingrich in glass and the surface of said buffer layer adjacent to saidlayer comprising electroluminescent phosphor being substantially free ofiron compound, and said buffer layer having a thickness at least aboutas great as the thickness of said layer comprising phosphor.

References Cited in the file of this patent UNITED STATES PATENTS3,007,070 Cargill Oct. 31, 1961

4. AN ELECTROLUMINESCENT CELL COMPRISING: A CONDUCTING SUBSTRATE; ASEMI-CONDUCTOR LAYER COMPRISING AN IRONTITANATE-CONTAINING MATERIALCARRIED ON SAID CONDUCTING SUBSTRATE; A TITANATE AND MIXED GLASS BUFFERLAYER COMPRISING ELE ON SAID SEMI-CONDUCTOR LAYER; A LAYER COMPRISINGELECTROLUMINESCENT PHOSPHOR CARRIED ON SAID BUFFER LAYER; AND ALIGHT-TRANSMITTING ELECTRODE LAYER CARRIED ON SAID LAYER COMPRISINGELECTROLUMINESCENT PHOSPHOR; SAID SEMI-CONDUCTOR LAYER HAVING A VERY LOWELECTRICAL RESISTANCE COMPARED TO THE ELECTRICAL RESISTANCE OF SAIDBUFFER LAYER; SAID SEMI-CONDUCTOR LAYER ALSO HAVING A THICKNESS AT LEASTEQUAL TO THE THICKNESS OF SAID BUFFER LAYER; SAUD BUFFER LAYERCOMPRISING FROM 80% TO 95% BY WEIGHT OF AT LEAST ONE FINELY DIVIDEDMATERIAL OF THE GROUP CONSISTING OF BARIUM TITANATE, STRONTIUM TITANATEAND BARIUM-STRONTIUM TITANATE, AND WITH FROM 20% TO 5% BY WEIGHT OFGLASS FUSED ABOUT THE PARTICLES OF THE MATERIAL OF SAID GROUP; THESURFACE OF SAID BUFFER LAYER ADJACENT TO SAID SEMI-CONDUCTOR LAYER BEINGRICH IN GLASS AND THE SURFACE OF SAID BUFFER LAYER ADJACENT TO SAIDLAYER COMPRISING ELECTROMUMINESCENT PHOSPHOR BEING SUBSTANTIALLY FREE OFIRON COMPOUND.